can't run msconfig

Hi,
I was using the msconfig Utility (System Configuration Utility) when I received this error:

An Access Denied error was returned while attempting to change a service. You may need to log on using an administrator account to make the specified changes.

This now comes up every time I try to access msconfig.exe. I am the system administrator on this computer. I am the only user of this machine. I am assuming there is a registry setting that is keeping me from accessing this program. I can access it in safe mode but not in standard mode. I tried replacing msconfig with one off the WinXp CD but I still get the error.
Does anyone know what I can do to regain access?

Thanks for your help, Don